The spinal column is a flexible column formed from a linear series of vertebral bones separated by intervertebral discs. These discs reduce friction between adjacent vertebrae and absorb compression forces applied to the spinal column. A vertebra includes an anterior body and a posterior arch that surrounds the spinal cord. Spinal nerves extend from each side of the spinal cord and exit the column at the vertebral foramen, which is formed by the posterior arch. Articular processes, including the superior articular process and the inferior articular process, are small flat projections on the surfaces of the arches.
There are four facet joints associated with each vertebra, and these joints interlock with adjacent vertebrae. In this manner, facets on opposing processes determine the range and direction of movement between adjacent vertebrae, hence the flexibility of the spinal column. The facet joints maintain spinal stability, protect the disc from excessive stress, and assist the discs in allowing motion and controlling shear forces. These joints are vulnerable to degenerative spinal disorders.
Degenerative disc disease is typically caused by a loss of disc space height, leading to a narrowing of the neural foramen and subsequent neural compression, and causing back and radicular pain. Instability of the posterior elements can lead to a condition known as spondylolisthesis, in which a vertebral body slips forward in relation to an adjacent vertebrae. This movement of the vertebral body narrows the foramen and results in painful pressure on the nerve roots.
Degenerative disc disease often may be resolved through a spinal fusion procedure during which an interbody implant is implanted between the bodies of two adjacent vertebrae. Such interbody implants may be formed from titanium, carbon fiber, allograft, or other suitable material including, but not limited to, biocompatible materials such as PEEK™, available from Invibio®. Implantation of a substitute implant is designed to reestablish normal disc height, provide immediate stability to the motion segment, and provide a matrix for fusion. When the implant grows into the existing bone, the fusion becomes solid and movement is eliminated at that level. A fusion procedure may also involve the surgical implantation of hardware, such as plates, screws or cages.
Posterior Lumbar Interbody Fusion (PLIF) and Anterior Lumbar Interbody Fusion (ALIF) are two surgical fusion techniques used to treat degenerative disc disease. Transforaminal Lumbar Interbody Fusion (TLIF) is another means of accessing the interbody space. TLIF involves the removal of one facet joint, usually on the more diseased or symptomatic side of the spine. Approaching the disc space and spinal canal from one side of the intervertebral space allows the surgeon to operate with minimal stretching of nerve roots.
In a typical procedure, the patient is positioned using a frame or spine table according to standard procedures, and the surgeon selects an appropriate approach angle based on patient pathology. After an incision is made, the correct level and facet joint are exposed visually and with x-ray verification. A facetectomy is performed and portions of the articular processes are removed as required, and neural elements are protected and/or retracted as necessary. After the disc is removed from the disc space, appropriate distraction is performed.
Depending on the patient's disc space, distraction can be performed with trials or with a paddle distractor. Distraction using modular trials is accomplished by threading individual trials onto the insertion instrument and placing them into the disc space sequentially until the desired disc height is achieved. Distraction using a paddle distractor is achieved by placing the distractor into the disc space and rotating 90 degrees.
Various kidney-shaped (or banana-shaped) implants have been designed to be impacted across the disc space to achieve arthrodesis. Although longer, straight implants have been placed across the disc space with some success, the lordotic angle of the spine is harder to properly match with these straight implants. The kidney-shaped implant helps maintain proper lordosis when it is placed in the anterior third of the disc space.
Proper alignment of the implant is imperative because improper positioning can lead to reduced fusion site stability, as well as improper balance of the spinal column. In some current procedures, proper positioning of a kidney-shaped implant in the disc space requires multiple x-ray images during insertion and placement. Such repeated x-ray imaging is disruptive and costly. Thus, a need exists for a system allowing visualization of proper alignment during the insertion and positioning process.